1. Field of the Invention
The present invention relates to a developing device used in an electrophotography or electrostatic recording type image forming apparatus such as a laser printer, a copying machine, or the like.
2. Related Background Art
As conventional electrophotography methods, many methods are known, as described in U.S. Pat. No. 2,297,691, Japanese Patent Publication Nos. 42-23910 and 43-24748, and the like. In general, an electric latent image is formed on a photosensitive member consisting of a photoconductive material by various means, and the latent image is visualized using a toner. The toner image is transferred onto a transfer member such as a paper sheet, as needed, and thereafter, the transferred toner image is fixed by heating, solvent vapor, or the like, thus obtaining a copy. Also, various methods of visualizing an electrostatic latent image using a toner are known.
As developing methods, a large number of developing methods, for example, a magnetic brush developing method described in U.S. Pat. No. 2,874,063, a powder cloud method and a fur brush developing method described in U.S. Pat. No. 2,221,776, a liquid developing method, and the like, are known.
Of these developing methods, in particular, a magnetic brush method, a cascade method, a liquid developing method, and the like, which use a developing agent consisting of a toner and carrier as major components, have been widely used in practical applications. These methods are excellent methods since a good image can be relatively stably obtained. However, they suffer from common drawbacks associated with a two-component developing agent, i.e., deterioration of the carrier, and a variation in mixing ratio of a portion A of the toner and carrier.
In order to avoid such drawbacks, various developing methods which use a one-component developing agent consisting of only a toner have been proposed. For example, U.S. Pat. No. 3,909,258 proposes a method of developing an image using a magnetic toner having conductivity. This method develops an electrostatic latent image by bringing it into contact with a conductive magnetic toner supported on a cylindrical conductive sleeve having a magnetic field formed therein. In this case, a conductive path is formed by toner particles on a developing region between the surface of a recording member and the surface of the sleeve, electric charges are guided from the sleeve to the toner particles via the conductive path, and the toner particles become attached to an electrostatic image portion by a coulomb force between themselves and the image portion, thereby developing the image. The developing method using the conductive magnetic toner is an excellent method since it can avoid conventional problems associated with a two-component toner. However, since toner has conductivity, it is difficult to electrostatically transfer a developed image to a final support member such as a normal paper sheet.
In order to solve this problem, as a developing method using a high-resistance toner, which can be electrostatically transferred, Japanese Laid-Open Patent Application No. 52-94140 discloses a developing method utilizing dielectric polarization of toner particles. However, this method essentially suffers from drawbacks such as low developing speed, insufficient density of the developed image, and the like, and it is difficult to use this method in practice. As another method using a high-resistance toner, a method of triboelectrically charging toner particles by friction between toner particles, friction between toner particles and a sleeve, and bringing the charged toner particles into contact with an electrostatic holding member to develop an image is known. However, these methods suffer from the following drawbacks. That is, the number of times of contact between the toner particles and the friction member is small, and triboelectric charging tends to be insufficient, or when the coulomb force between the charged toner particles and the sleeve is strong, the toner particles tend to agglomerate. For these reasons, many practical difficulties arise.
In contrast to this, Japanese Laid-Open Patent Application No. 54-43036 proposes a novel developing method which eliminates the above-mentioned drawbacks. In this method, a very thin toner layer is coated on a developing sleeve, and is triboelectrically charged. Then, the charged toner layer is brought very close to an electrostatic image to face the image without contacting it under application of a magnetic field, thereby developing the image.
According to this method, since a very thin magnetic toner layer is coated on the developing sleeve, the number of contact chances between the magnetic toner and the developing sleeve increases, and a triboelectrically charged electric charge amount required for development can be given to the toner.
In the developing method using a magnetic toner, as described in, e.g., Japanese Laid-Open Patent Application No. 54-43036, the thickness of a toner layer on the developing sleeve can be regulated to a predetermined toner layer thickness by a magnetic field formed between the developing sleeve and a magnetic doctor blade which is arranged to face an internal magnet of the developing sleeve.
However, the above-mentioned one-component system developing methods using the magnetic toner suffer from some drawbacks. For example, the fixing characteristics upon thermal fixing of a toner image transferred onto a transfer sheet are poor, and it is impossible to use color toners since the toner itself contains a magnetic member.
Contrary to this, as one-component developing methods using a non-magnetic toner, a powder cloud method, an impression developing method, and the like are known. Of these methods, a fur brush developing method for developing an image with a developing agent attached to a cylindrical brush formed using a soft fur of, e.g., a beaver, an impression developing method using a developing agent attached to, e.g., a velvet roller, and the like are well known. These developing methods do not use a toner, to which a magnetic substance is internally added, upon development of an electrostatic latent image, and does not require any complex device for maintaining a constant ratio between toner and carrier particles.
However, the developing method using a non-magnetic toner cannot obtain a predetermined uniform toner layer even when the same magnetic blade as that in Japanese Laid-Open Patent Application No. 54-43036 is used. For this reason, a nonuniform toner layer is coated on the surface of the developing sleeve, and this appears as image density nonuniformity. When an elastic blade is used as the doctor blade, the toner layer thickness can be regulated. In this case, the latitude of a pressing pressure of the elastic blade against the developing sleeve is very narrow. For this reason, when the pressing pressure is too low, a predetermined uniform toner layer thickness cannot be obtained; when it is too high, the toner and the developing sleeve are damaged.
In order to solve the above-mentioned drawbacks, developing methods described in, e.g., Japanese Patent Publication Nos. 58-90668, 58-143360, and the like are known.
FIG. 5 shows an example of a developing device for practicing such a developing method. Referring to FIG. 5, a developing device 20 comprises a toner container 3 for storing a non-magnetic toner 11 and magnetic particles 10, a developing sleeve 1a which consists of a non-magnetic member, faces a photosensitive drum 15 as a latent image carrier, and is arranged at the opening portion of the toner container 3, a permanent magnet 1b fixed in the developing sleeve 1a, and a magnetic member blade 2 which is fixed to the toner container 3, and regulates the thickness of a toner layer which is supported and conveyed toward the photosensitive drum 15. Note that the interval or gap between the magnetic member blade 2 and the developing sleeve 1a is set to be 250 .mu.m.
In the developing device 20 with the above-mentioned arrangement, the magnetic particles 10 form a brush 10a along a magnetic field between the magnetic member blade 2 and the permanent magnet 1b, and are restricted in the magnetic field. The non-magnetic toner 11 moves while being dragged by the developing sleeve 1a having a roughened surface, is simultaneously charged by friction between the magnetic particles and the developing sleeve 1a, and becomes attached to the magnetic particles and the developing sleeve 1a due to a reflection force. However, since the magnetic particles cannot move due to restriction of the above-mentioned magnetic field, only non-magnetic toner particles attached to the developing sleeve 1a can pass through the brush formed by the magnetic particles, and a uniform non-magnetic toner layer is formed on the surface of the developing sleeve 1a upon rotation of the developing sleeve 1a.
In the above-mentioned conventional developing device, the magnetic particles must perform the functions of forming a magnetic brush in a system in which a large quantity of toner are present, coating the toner onto the developing sleeve, and regulating the coating amount of the toner. At the same time, the magnetic particles must perform the function of supplying the toner while cyclically moving, and it is not preferable for the magnetic particles to pass between the magnetic member blade and the developing sleeve. Therefore, in order to satisfy these functions, the magnetic particles must exhibit a proper circulation property while maintaining a proper restriction force generated by the magnetic field, and must form a magnetic brush which has a proper hardness and density to allow uniform coating.
For example, a relatively coarse brush tends to form a stripe indicative of insufficient regulation on the developing sleeve. On the other hand, a relatively dense brush tends to extremely decrease the thickness of the toner layer on the developing sleeve.
When the circulation property of the magnetic particles is too high, the thickness of the toner layer increases to cause fogging on an image, and the magnetic particles undesirably pass through the magnetic member blade. On the other hand, when the circulation property is poor, the magnetic particles cannot sufficiently charge the toner. For this reason, an image error called a ghost phenomenon tends to occur, i.e., the density of an image formed by the first revolution of the developing sleeve is high, but that of images formed by subsequent revolutions is low.
In other words, the latitude of a proper state of the brush which can satisfy the functions of the magnetic particles is very narrow.
Various developing methods which use a one-component developing agent consisting of only a toner have been proposed. For example, U.S. Pat. No. 3,909,258 proposes a method of developing an image using a magnetic toner having conductivity. This method develops an electrostatic latent image by bringing it into contact with a conductive magnetic developing agent supported on a cylindrical conductive sleeve having magnetism therein. In this case, a conductive path is formed by toner particles on a developing region between the surface of a recording member and the surface of the sleeve, electric charges are guided from the sleeve to the toner particles via the conductive path, and the toner particles become attached to an electrostatic image portion by a coulomb force between themselves and the image portion, thereby developing the image. The developing method using the conductive magnetic toner is an excellent method since it can avoid conventional problems associated with a two-component toner. However, since a toner has conductivity, it is difficult to electrostatically transfer a developed image to a final support member such as a normal paper sheet.
In order to solve this problem, as a developing method using a high-resistance toner, which can be electrostatically transferred, Japanese Laid-Open Patent Application No. 52-94140 discloses a developing method utilizing dielectric polarization of toner particles. However, this method essentially suffers from drawbacks such as low developing speed, an insufficient density of the developed image, and the like, and it is difficult to use this method in practice. As another method using a high-resistance toner, a method of triboelectrically charging toner particles by friction between toner particles, friction between toner particles and a sleeve, and bringing the charged toner particles to an electrostatic holding member to develop an image is known. However, these methods suffer from the following drawbacks. That is, the number of times of contact between the toner particles and the friction member is small, and triboelectric charging tends to be insufficient, or when the coulomb force between the charged toner particles and the sleeve is strong, the toner particles tend to agglomerate. For these reasons, many practical difficulties are pointed out.
In contrast to this, Japanese Laid-Open Patent Application No. 54-43036 proposes a novel developing method which eliminates the above-mentioned drawbacks. In this method, a very thin toner layer is coated on a developing sleeve, and is triboelectrically charged. Then, the charged toner layer is brought very close to an electrostatic image to face the image without contacting it under application of a magnetic field, thereby developing the image.
According to this method, since a very thin magnetic toner layer is coated, the number of contact chances between the magnetic toner and the developing sleeve increases, and a triboelectrically charged electric charge amount required for development can be given to the toner.
The present inventors examined electric charge application of the one-component developing method, and found that toner particles behaved as follows in the electric charge application portion of the one-component developing method.
FIG. 6 shows an example of a developing device using the magnetic one-component toner.
Referring to FIG. 6, the developing device comprises a toner container 3 for storing a magnetic one-component toner, a developing sleeve 1a which consists of a non-magnetic member, and is arranged at the opening portion of the toner container 3 to be rotatable in the direction of an arrow in FIG. 6, a permanent magnet 1b fixed in the developing sleeve 1a, a magnetic blade 2, consisting of a magnetic member, for regulating the thickness of a toner layer, and a toner convey member 4 arranged in the toner container 3. Note that the magnetic blade 2 is arranged to be separated from the developing sleeve 1a by a predetermined distance W. In general, the distance W is often set to fall within a range of from 100 .mu.m to 1 mm.
In the developing device shown in FIG. 6, a magnetic one-component toner thin layer is coated on the developing sleeve 1a. The thickness of the toner layer is determined by the position of a cut line L shown in FIG. 8.
As a result of our study, it was found that a magnetic toner T was charged when the magnetic toner T passed between the developing sleeve 1a and the magnetic blade 2. Also, it was found that the behavior of the magnetic toner at that time was as follows.
As shown in FIG. 7, planes perpendicular to a straight line connecting the developing sleeve 1a and the magnetic blade 2 are assumed, the plane closer to the magnetic blade 2 is represented by S1, and the plane closer to the developing sleeve 1a is represented by S2. The width of the magnetic blade 2 is normally set to be smaller than that of the permanent magnet 1b. In this case, upon examination of the magnetic flux densities on the planes S1 and S2, the magnetic flux density on the plane S1 is larger than that on the plane S2. Therefore, the magnetic toner T receives a force in the direction of an arrow in FIG. 7, i.e., a force toward the magnetic blade 2 side, between the developing sleeve 1a and the magnetic blade 2.
Therefore, as shown in FIG. 8, the magnetic toner particles T form ears (state B), and these ears are formed from the magnetic blade 2 in the direction of the developing sleeve 1a. In this case, the magnetic toner T is charged as follows. That is, when toner particles t1 at the distal ends of the ears formed from the magnetic blade 2 contact the developing sleeve 1a, an electric charge is applied to the distal ends of the ears.
It was also found that the toner was conveyed as follows between the developing sleeve 1a and the magnetic blade 2.
As described above, since an electric charge is applied to the toner particle t1 at the distal end of each ear contacting the developing sleeve 1a, a force in the direction of the developing sleeve 1a due to a reflection force acts, and a convey force in the rotational direction of the developing sleeve 1a is applied to the toner due to a frictional force with the developing sleeve 1a.
Since an agglomeration force acts between the toner particles to some extent, a convey force is also applied to a toner particle t2, which contacts the toner particle t1, via the agglomeration force. Furthermore, a convey force is similarly applied to a toner particle t3 in an upper layer portion via the agglomeration force.
However, a magnetic force in the direction of the magnetic blade 2 is applied to the toner between the developing sleeve 1a and the magnetic blade 2, as described above. Therefore, a portion A of the ear of toner is torn off at a position where the convey force acting on the toner overcomes the magnetic force, i.e., at the position of the cut line L shown in FIG. 8, and toner particles remaining on the developing sleeve 1a are conveyed in the rotational direction of the developing sleeve 1a.
Therefore, in a system having a high degree of agglomeration of a magnetic toner, a system which requires a large number of times of contact to obtain a necessary triboelectric charging electric charge amount, and the like, insufficiently charged toner particles which do not contact the developing sleeve are undesirably conveyed to a developing region, and a phenomenon caused by a charging error tends to occur.